Inorganic mineral scale concretion or the deposition of scale coatings derived from the extended contact of surfaces normally susceptible to scale formation with hard waters, particularly waters containing alkaline earth and/or iron compounds, is both wasteful and commonplace. While scale formation upon metal surfaces presents the more serious problem, non-metallic surfaces including ceramics, glasses and organic polymers may also undergo scaling.
Particularly troublesome are the tenacious "furs" or "crusts" that form in operational processing equipment in continuous and/or prolonged contact with aqueous solutions of inorganic salts as, for example, alkaline earth sulfates, carbonates and bicarbonates. This type of scale formation creates serious problems for the petroleum and gas-producing industries, where the build-up of undesirable inorganic mineral scale deposits can take place in equipment or tubing of difficult or limited accessibility. These include flow lines in both surface and subterranean locations, including auxiliary producing equipment, such as heaters, pumps, valves, rods, and the like. In addition, scale formation can also cause substantial problems in the chemical processing industries, public utilities, and in other situations where mineral-laden water is processed or used, as in heat exchangers, storage vessels, piping, reactors, evaporators, and the like.
Several different explanations have been advanced as to how the formation of mineral scale deposits, such as for example calcium or barium sulfate, takes place. One mechanism leading to scale formation comes about from the mingling of a fluid stream containing a substantial concentration of an anion or cation with another stream containing antagonistic counter ions. An illustration of this mechanism would be a case in which one process stream containing a soluble calcium compound contacts another stream containing a soluble sulfate compound to cause precipitation of calcium sulfate. Should the mixing of such incompatible waters take place in a well bore, the result may be deposition of a crust of calcium sulfate which, if allowed, can build up on the surface of submerged equipment such as tubing, and may even choke off fluid flow by diminution of utilizable diameter unless corrective descaling measures are undertaken. In the case of heat exchange equipment, the mineral scale deposition insulates the equipment from the source or sink of heat and increases the cost of operation and may cause extensive maintenance or down-time and increased operational costs.
Another cause of mineral scale formation arises from aqueous solutions of inorganic materials having an inverse solubility curve, that is, a solubility which decreases as the temperature increases. An excellent example of this is calcium sulfate, whose solubility in water decreases with increased temperature. In these instances the solution immediately adjacent to the heating surface reaches saturation the most quickly, dropping calcium sulfate at this juncture which is held tenaciously to the heating surface.
Yet another cause of scale formation is attributable to precipitation of scale material from supersaturated solutions, of which the preceding example is a special case. When temperature and/or pressure changes occur, or the concentration of some solubilizing substances is substantially decreased, this change of conditions can result in scale formation on the tubing or other equipment being operated.
Whatever the reason or reasons for scale formation may be, as outlined above, it is a troublesome and expensive process that can lead to costly, unscheduled maintenance and even to the breakdown of operational units. For these reasons, a number of remedial measures have been resorted to, both for the removal of scale and mitigation of its formation.
Scale is ordinarily removed by either chemical or mechanical methods, or a combination of both methods. In chemical descaling procedures, the problem is to find a material that will dissolve or loosen the mineral scale without attacking the underlying substrate.
Inhibited hydrochloric acid has proved useful in instances where the scale is acid soluble, as is the case with calcium carbonate scale. Unfortunately, however, many mineral scales, such as those of calcium and barium sulfates, are not appreciably soluble in acids. Calcium sulfate scale can sometimes be dissolved or loosened by treatment with ammonium salts, polyphosphates, or hydroxides, often followed by acid washing.
Mechanical descaling procedures are often resorted to employing such devices as scrapers, brushes, high-pressure abrasive jets, and the like, but these methods are tedious, expensive, and of restricted applicability.
In view of the numerous difficulties inherent in removal by whatever means of mineral scale once formed, the use of scale-preventive methods has been widely practiced for many years. The addition of chemical "scale inhibitors" such as phosphates, both organic and inorganic, tannin materials, chelating agents, natural and synthetic polymers, and the like to scale-prone systems is therefore old in the art. However, the use of such scale inhibitors also suffers serious shortcomings, the most notable being (1) the necessity of maintaining them at constant critical levels of concentration in the systems being protected, (2) the absence of chemical scale inhibitors which provide protection under a diversity of scaling conditions, (3) their general thermal lability and (4) their tendency after prolonged usage to themselves induce the formation of difficult soluble deposits.
As a consequence of the many problems, as outlined in brief above, which are encountered in methods of removing scale already deposited, as well as in the methods of scale prevention as currently practiced, a novel and efficient means is taught by the instant invention whereby the deposition and/or adherence of mineral scale from hard waters upon surfaces may be mitigated or prevented by virtue of a novel protective coating which is at once long lived, environmentally stable, requires little maintenance, and is broad in applicability.
Various types of protective "plastic" coatings are known for use as surface barriers to liquid penetration, weather and the like. For example, U.S. Pat. No. 3,499,783 describes a polyurethane coating used as a protective barrier. However, this disclosure fails to teach anything about scale deposition resistance.
U.S. Pat. No. 3,502,587 discloses scale inhibitor additives which are phosphate esters containing oxyethylene groups. This, of course, is not a coating but an additive to aqueous solutions. Coatings are not mentioned. Oxyethylene groups are taught, but they are also taught in U.S. Pat. No. 3,499,783, above, and impart no scale resistance to that conventional coating. Therefore, the presence of oxyethylene groups is not determinative and does not lead one to our invention to be disclosed herein.
Other disclosures in the art not related to inhibiting of inorganic scale are available. For example, U.S. Pat. No. 3,249,535 teaches a method of using a silioxane copolymer to inhibit paraffin deposition in conduits. Since the present invention is concerned with inorganic mineral scale only, not organic paraffin; and since silioxane compounds are unrelated to the compositions of our invention, nothing is taught in this patent to lead one to our invention.
It should be noted that the coatings of the instant invention are not restricted in applicability simply to metallic surfaces, which may be susceptible both to corrosion and scaling, but are also applicable to non-metallic surfaces which may be inherently resistant to corrosion but scale-prone, such as glass, ceramics, organic polymeric materials, and the like.
In view of the foregoing evidence, it was unexpected to find that conventional, scale-prone, polyurethane-derived protective resins, even those originally compounded with a polyether polyol as the reactive hydroxylic component, if modified in their formulation so as to contain in the finished or "cured" resin pendant polyoxyalkylene chains, will become resistant to mineral scale deposition and/or adherence.
Thus, it is a broad object of this invention to provide novel, modified polyurethane resin coatings which when applied to metallic or nonmetallic substrates normally susceptible to the build-up of scale, greatly inhibit scale deposition and/or adherence thereon as compared to the uncoated substrate.
A further and more specific object of this invention is to provide processes whereby the scale-susceptibility of scale-prone surfaces is substantially lowered by application thereto of novel, modified polyurethane-type resins which possess scale inhibitory properties, and curing said resins on the said surfaces.
Yet a further and more specific object of this invention is to provide compositions of novel, modified polyurethane resins containing pendant polyoxyalkylene chains and having substantial resistance to scale deposition and/or adherence thereon.
Additional objects of this invention are the preparation from novel, modified polyurethane-type resins of scale-resistant articles of manufacture such as conduits, membranes, vessels, and the like which may be subject to service in the presence of mineralized waters.
Other objects of this invention will become apparent to those skilled in the art after a perusal of this disclosure.
The above objects are achieved through the utilization of several conceptual variations including the following:
I. The preparation of novel polyurethane resins which are highly resistant to scale deposition and/or adherence during contact with mineralized waters.
II. The use of novel polyurethane resins as coatings to impart scale resistance to substrate surfaces, which are normally scale-prone, during contact with mineralized aqueous environments.
III. The production of novel, scale-resistant, polyurethane-coated items of service such as piping, heat exchangers, storage and mixing vessels, chemical reactors, and the like which uncoated, would normally be susceptible to scale formation and/or adherence.
IV. The production from novel polyurethane resins, of scale-resistant fabricated articles of manufacture such as containers, conduits, membranes, filaments, and the like, which may be subject to usage in the presence of scale-forming mineralized waters.
V. The production from novel polyurethane resins, of fabricated articles of manufacture whose usefulness lies in applications apart from scaling problems and is due to their increased attraction and/or retention of water molecules.
The foregoing innovations comprising the gist of this invention are outlined more particularly below.
In the first compositional aspect of this invention novel polyurethane resins resistant to scale deposition are prepared by:
A. Admixing polyisocyanates, preferably aromatic polyisocyanates, with polyols having (1) a functionality of three or greater, and (2) having been prepared so as to contain polyoxyalkylene chains made up of oxethylene linkages, oxypropylene linkages, and/or combinations thereof, to form resin mixtures, said polyols being present in excess over what is required by stoichiometry to convert all of said isocyanate groups to urethane groups thus providing the requisite, pendant polyoxyalkylene chains. Usually it suffices for about 1.1 to 1.9 hydroxyl equivalents to be present per equivalent of isocyanate, with 1.3 to 1.7 being preferred, and
B. Partially curing the said resin mixtures at temperatures and times sufficient to produce enough crosslinking to provide polymeric networks without sufficiently "setting" the resins to such an extent as to prevent their being applied by conventional means as coatings or being formed into fabricated articles, and without significant alteration of the integrity of the pendant polyoxyalkylene chains.
C. Applying to the surface or article to be protected from scaling, a coating of the partially cured polyurethane resin mixture by spraying, painting, dipping, or other means, and further curing the applied resin to a degree such that the resin possesses good adhesion to the substrate, stability to in-service environmental conditions, but without significantly altering the integrity of the pendant polyoxyalkylene chains in the resin.
In the second compositional aspect of this invention, novel polyurethane resins resistant to scale deposition are prepared by:
A. Admixing isocyanates, preferably aromatic polyisocyanates, with (1) an amount of conventional polyol substantially less than that stoichiometrically required for complete reaction with said polyisocyanates, said polyol having an average hydroxyl functionality of three or greater per molecule, and (2) a compound characterized by a polyoxyalkylene chain terminated at one end by an alkyl, aryl, or other radical which is nonreactive with isocyanate functional groups, and terminated at the other end by a hydroxyl group which, upon reaction with the isocyanate groups in the mixture present in excess of those required for complete reaction with the polyol component, ultimately provides the requisite pendant polyoxyalkylene chains in the mixture.
B. Partially curing the said resin mixtures at temperatures and times sufficient to produce enough crosslinking to provide polymeric networks without sufficiently "setting" the resins to such an extent as to prevent their being applied by conventional means as coatings or being formed into fabricated articles, and without significant alteration of the integrity of the pendant polyoxyalkylene chains.
C. Applying to the surface or article to be protected from scaling, a coating of the partially cured polyurethane resin mixture by spraying, painting, dipping, or other means, and further curing the applied resin to a degree such that the resin possesses good adhesion to the substrate, stability to in-service environmental conditions, but without significantly altering the integrity of the pendant polyoxyalkylene chains in the resin.
It should be noted that in this second compositional variation of the resins of the instant invention, the free end of the pendant polyoxyalkylene chain is terminated by a group less labile to in-service degradative chemical attack than are the terminal hydroxyl groups characteristic of the pendant chains of the first compositional variation herein above described.
In order to further supplement the understanding of the invention and to disclose the invention in close detail, the following additional information is submitted: